Ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with energized ions. In semiconductor manufacturing, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses and at different energy levels.
To form devices on a semiconductor wafer, it is usually necessary to implant impurities at different depths of the semiconductor wafer. The energy of impurities in an ion beam directed toward the semiconductor wafer is determinative of the depth to which the impurities penetrate into the semiconductor wafer. As devices are reduced in size and increased in speed, it has become desirable to use very low energy ion beams to form, for example, shallow transistor junctions in the semiconductor wafer.
At the same time, high energy ion implantation may help minimize production costs because high energy ion implantation does not require some conventional processes, such as, but not limited to, the masking of a semiconductor wafer, to be performed. Also, semiconductor devices manufactured through the use of high energy ion implantation may exhibit relatively low levels of junction leakage and improved latch-up characteristics. Thus, the production yield may be high with respect to an ion implantation process carried out by high energy ion implantation. Therefore, high energy ion implantation may be widely used for implanting ions in semiconductor device manufacturing processes.
FIG. 1 depicts a prior art ion implanter system 100. The ion implanter system 100 may comprise an ion source 102 and a complex series of components through which an ion beam 10 passes. The series of components may include, for example, an extraction manipulator 104, a filter magnet 106, an acceleration or deceleration column 108, an analyzer magnet 110, a rotating mass slit 112, a scanner 114, and a corrector magnet 116. The ion source 102, the extraction manipulator 104, and the filter magnet 106 may be housed in a terminal 118. Much like a series of optical lenses that manipulate a light beam, the ion implanter components can filter and focus the ion beam 10 before steering it towards an end station 120. The ion implanter components that can filter and focus the ion beam 10 may be referred to as optical elements, or beam optics.
The acceleration or deceleration column 108 is a critical component to the ion implanter system 100. As the range of required energy level for ion beam may be wide (e.g., from about 1 kV to above 600 kV), the acceleration or deceleration column 108 may be required to accelerate or decelerate ions to a wide voltage spectrum (e.g., from about 1 kV to above 600 kV).
Conventionally, a resistive divider may be used to gradually accelerate (divide the acceleration potential) or decelerate (divide the deceleration potential) the ion beam along the column 108. That is, one or more resistors may be electrically connected between adjacent electrodes along the column 108. A plurality of resistors may thus form a chain of resistors. Each electrode of the column 108 may be electrically connected to certain electrical contacts along the resistor chain. Thus, the acceleration voltage or deceleration voltage may be distributed by the resistors. The distribution of voltage along the column 108 may be referred to as grading. As the operational voltage spectrum gets wider and upper voltage gets higher, there may be at least two concerns with the conventional resistive divider.
The first concern with the conventional resistive divider in wide range voltage ion implantation may be ballast current. Electrical current flowing through a conventional resistive divider may be referred to as divider current. The divider current sometimes may be referred to as ballast current as it may facilitate the maintenance of a voltage gap between electrodes of the column 108.
For high energy acceleration, beam rigidity may be high. That is, amount of magnetic force or electrostatic force required to deflect a beam may be high. In the mode of high energy acceleration, most ions of an ion beam will not stray away from the direction the ion beam is going and will pass through an aperture of the electrodes of the column 108. Because the ions are charged particles, the transportation of the ions may have an ion beam current. So, for high energy acceleration, there is little interception of ions by the electrodes of the column 108. The ion beam current may have very little loss. The electrodes are not going to charge up since the ion beam is well focused and stable operation can be achieved. Thus, divider current flowing through the chain of dividing resistors may be relatively low (e.g., hundreds of micro amperes), but may still maintain the voltage gap between electrodes of the column 108. Therefore, a relatively low ballast current may be required for high energy acceleration.
When the acceleration voltage goes down (e.g., low to medium energy acceleration or deceleration), the divider current may be in low micro-amperes (i.e., V/R is lower). In this case, low current may perform grading. However, beam rigidity may also be lower compared with high energy acceleration. That is, compared to a high energy mode of operation, more ions may stray away from the ion beam and get intercepted by the electrodes of the column 108. In certain instances, the ion beam may generate secondary ions/electrons that may intercept the electrodes of the column 108. When sufficient ions/electrons intercept the electrodes of the column 108, latch-up may occur. That is, the divider current may be insufficient to maintain the voltage gap between two electrodes and can lead to voltage collapse. Latch-up may cause focus degradation and eventually lead to an unrecoverable situation due to the electrodes of the column 108 changing voltage from ion/electron current. Very similar instability may occur for the deceleration.
Generally, the lower the ion beam energy, the higher the current flowing through the resistor chain may be required. Therefore, much higher ballast current may be required for low energy acceleration and deceleration than for the high energy acceleration. This is practically impossible to accomplish by using just a linear single resistor chain. For example, when the beam has a low energy, the Ohms law dictates a low resistance to maintain a sufficient current (I=V/R). However, when the beam has a high energy, a high voltage may be necessary. Power generated may be high if both current and voltage are high (P=IV=I2R=V2/R). A lot of heat may be generated by the current. The resistors may get extremely hot and their resistance changes.
One possible solution to the above described problems has been to use switches to select a different resistor chain with lower resistance for low energy acceleration and deceleration. The switches may be activated electrically and may be referred to as electrical switches. The electrical switches may be activated by relays, for example. The switches may also be powered by pressurized fluid and may be referred to as hydraulic/pneumatic switches. The hydraulic/pneumatic switches may be activated by pressurized media in a conduit made of dielectrical material (e.g., polytetrafluoroethylene (PTFE) air lines). However, for low resistance resistors, the ballast current typically becomes very high and generates a lot of heat on the resistors, possibly leading to over-heating and thermal failure. To avoid this from happening, proper switching sequence depending on acceleration voltage may be chosen.
The second concern with the conventional resistive divider in wide range of voltage for ion implantation may be high electric field stress that each resistor may be required to be protected against. In a high energy mode, a high acceleration voltage (e.g., above 650 kV) may be graded over the resistor chain. Conductive materials of the resistors, such as lead or ferrule connections, may need shielding due to high electric field stress. Such shielding has previously been achieved by placing the resistor chain in a pressurized vessel that may be filled with a high dielectric strength material (e.g., SF6 gas). However, service of the resistor chain is not easy in such configuration as the pressurized vessel needs to be drained, which may be slow and labor intensive.
In view of the foregoing, it may be understood that there are significant problems and shortcomings associated with current technologies in controlling an accelerated or decelerated charged particle beam for ion implanters.